The present invention relates generally to a system for stimulating bone growth and tissue healing, and more particularly to a method and apparatus for stimulating bone growth and tissue healing by applying an electrical current to the bone and adjacent soft tissue through a partially insulated screw.
Bone growth is desirable in many instances, such as when vertebrae in a patient's spine are fused to overcome pain and other effects caused by inter-vertebral movement or intra-vertebral movement. Although bone growth occurs naturally, it can be stunted or stopped by various factors such as tobacco, alcohol and steroid usage, poor bone stock, and age. Moreover, stimulating bone growth to speed recovery is desirable in some instances, such as when an injured athlete wishes to return to her sport quickly. Other motivations for stimulating bone growth are to reduce chronic pain, to improve mobility, and avoid future complications. Thus, there is a need for stimulating bone growth in individuals.
Bone growth, tissue healing and pain control can be stimulated by various means. One such means for stimulating bone growth, tissue healing and pain control is by passing an electrical current through the bone. As one example, when fusing vertebrae in a patient's spine, various means have been used to stimulate bone growth. For example, some stimulators include wire electrodes embedded in bone fragments grafted to a region of the patient's back containing the vertebrae to be fused. Direct or alternating electrical current is applied to the electrodes to stimulate bone growth and fuse the fragments and adjoining vertebrae. To permit the current to be applied for extended periods of time while permitting the patient to be mobile, a generator is connected to the wire electrodes and implanted between the skin and muscle near the patient's vertebral column. The generator provides a continuous low amperage direct current (e.g., 20-100 μA) for an extended period of time (e.g., six or more months). After the vertebrae are fused, the generator and leads are surgically removed. Although these embedded electrodes are generally effective, the wire electrodes are susceptible to failure, requiring additional surgery to repair them. Moreover, placement of the wire electrodes is less than precise, allowing some of the current to pass through areas of tissue and bone where it is unneeded and where the current could potentially have adverse effects. Further, due to imprecise placement or lack of proximity to an area of interest, more energy must be provided to the electrodes than otherwise is necessary to be optimally effective. Thus, there are several drawbacks and potential problems associated with devices such as these.
Although small amounts of mechanical loading can stimulate growth, it is generally desirable to limit movement between the bones or bone fragments being fused. There are several known means for limiting bone movement. Among these means for limiting bone movement are plates, rods and screws. The plates and rods are typically held in position by screws which are mounted in the bone or bones being fused. FIG. 1 illustrates screws (generally designated by 10) driven into a vertebra 12 to immobilize the vertebra. As previously mentioned, the screws 10 are used for attaching rods 14 and/or plates (not shown) to vertebrae to hold the vertebrae in position while they fuse. Although these screws 10 work well for their intended purpose, they only provide mechanical fixation, and do not provide other potential benefits, such as facilitating electrical stimulation of the region and lack of adapting with changing tissue environments. In addition, with such conventional screws, undesirable complications may include loosening over time; being prone to pullout; being prone to infection; and not being useful in degraded osteoporotic or compromised bone. Moreover, if electrical stimulation were applied to bones using conventional screws, the screws 10 would not focus therapeutic stimulation and bone growth to anatomical areas where it is most desired and/or needed. Rather, they could potentially conduct current to areas of tissue and bone where the current is unneeded and where the current could potentially have adverse effects. Thus, there are drawbacks and potential problems associated with conventional screws such as these.
Beyond the well-defined role of electrical fields within bone formation, electrical fields have also shown significant promise in aiding healing and recovery in nerve and spinal cord injury. Stimulating tissue healing with electrical currents has been demonstrated to be efficacious in animal models and is now being attempted experimentally in human subjects. Further, spinal cord and nerve root injury has been known to cause associated debilitating pain syndromes which are resistant to treatment. These pain syndromes also have shown improvement with pulsed electrical stimulation. Given these findings, it is envisioned that a system and/or an apparatus providing a specified and confined electrical field through bony constructs and adjacent tissue (e.g., neural tissue) will facilitate an enhanced recovery from spinal cord and nerve injury, including improved functional outcome, better wound healing, and a higher level of pain control.
In U.S. Pat. No. 3,918,440, Kraus teaches the use of alternating current (A.C.) for assisting in the healing of bone damage. A.C. current carries several disadvantages. A.C. current relies on a complex power supply. In addition, it is more difficult to predict and control the spatial distribution of A.C. current within a body, since current may flow both through resistive and capacitive paths. Overall, substitution of D.C. for A.C. current results in system simplifications and opportunity to improve precision in targeting treatment to particular areas of interest within the body, while avoiding collateral damage to surrounding tissues. D.C. current is potentially advantageous in that required energy can be provided by batteries. However, it is critically important to properly size the battery powering a D.C. stimulation system to prevent premature interruption of the scheduled treatment. In fact, engineering tradeoffs include at least battery size, voltage, amp-hours, self-discharge rate, cost, and form factor. Clearly, there is a need for a D.C. stimulation system that optimally conserves power and allows for stimulation of bone growth and tissue healing. A smaller, lower cost battery will lead to increased patient mobility and comfort.